Magnet and method of forming uniform magnetic field for MRI system

ABSTRACT

The present invention relates to the magnetic apparatus for magnetic resonance imaging system and method for forming uniform magnetic field. The magnetic apparatus comprises a field yoke for forming flux path, a first member of permanent magnet, provided on one end of the field yoke, free end of which is inclined inwardly, a second member of permanent magnet, provided on the other end of the field yoke in symmetry with the first member of permanent magnet, having reverse polarity with the first member of permanent magnet, an adjusting block, provided on the center of the field yoke for extruding magnetic field formed by the first and second members of permanent magnet. The adjusting block in the present invention can form a magnetic field for adjusting to extrude the basic magnetic field in order to provide a parallel magnetic field. The present invention can be applied in the MRI system and make the system have a high open degree for operations. The MRI system can be made compact, movable and inexpensive.

FIELD OF THE INVENTION

The present invention relates to a permanent magnet for MagneticResonance Imaging (MRI) system, and particularly to a permanent magnetfor imaging in a sheet of sensitive field for MRI system and method offorming uniform magnetic field in a sheet.

BACKGROUND OF THE INVENTION

The conventional Magnetic Resonance Imaging (MRI) scanner cannot beapplied in the interventional therapy to the extent that the magnetsurrounds the patient and the access to the patient is obstructed;meanwhile, a highly uniform magnetic field is required within a sphereof 30˜50 cm diameter and the patient can pass in and out the spherefreely in the conventional MRI scanner, which lead to a large and heavymagnet. A compact and movable MRI system cannot be made.

The above-mentioned two disadvantages of the conventional MRI scannerresult from the conventional imaging method. According to the method,the positions of the patient to be examined (sample for short) such ashead, thoracic cavity and abdominal cavity etc., must be first entirelyplaced in a highly uniform magnetic field (the inhomogeneity is withinthe order of 10⁻⁵), which is generally a sphere of 30˜50 cm diameter.Then, by adding the slice selection gradient field, the different tissueslices of the positions to be examined which are entirely in the highlyuniform magnetic field, along the direction of the gradient field, arerespectively controlled by the magnetic fields of different magneticintensity. According to the Nuclear Magnetic Resonance Theory, with theradio frequency (RF) field of different temporal frequency, the humanbody tissue slices can be excited respectively at this time; that is,with the radio frequency (RF) field at the Larmor frequency of thenucleus to be examined corresponding to the magnetic intensity of theset objective slice to be excited, all the spins in the selectedobjective slice are excited. This progress is called imaging sliceselection by the gradient field, which is slice selection for short. Theslice to be excited is called objective slice. After the objective sliceis excited and the operations to the gradient coils, radio frequencysource and spectrometer by the suitable pulse sequence are completed,the magnetic resonance information of the excited slice is obtained.Finally, the magnetic resonance images are obtained by dealing with theinformation. Repeat all the steps above by changing the frequency of theradio frequency (RF), another selected slice is excited, and the imagesare obtained following the same steps as the above-mentioned. After allthe images for the set objective slices of the tissues to be examinedare obtained, the examination is finished.

The precondition of such imaging method is to acquire a highly uniformmagnetic field in a sphere of 30˜50 cm diameter with the inhomogeneityno more than 10⁻⁵ order. To this end, the volume of interest should bein the center of the magnet which surrounds the whole body of thepatient. Thus, the magnet is large and heavy and the interventionaltherapy is impossible. A compact and movable MRI system cannot be made.

An imaging method for MRI was presented by the applicant on the basis ofhighly open magnet design (referring to the CN patent No. 1371000A). Asshown in FIG. 1, the method includes: (1) constructing a volume ofinterest with a uniform magnetic field in a sheet outside the magnet;(2) moving the objective slice of the tissue to be examined to theposition overlapping with the sheet of interest by a servocontrolsystem; (3) exciting the objective slice with a RF field produced by aRF coil; (4) encoding the spins in the sheet with the gradient fieldalong two orthogonal directions except for the slice thickness directionproduced by gradient coils; (5) receiving the signal of the objectiveslice and reconstructing the image; (6) making another objective sliceoverlap with the sheet of interest by servocontrol system; (7) repeatingthe steps from (3) to (6) to obtain the image of another objectiveslice; (8) repeating steps from (6) to (7) until all the objective sliceimages are obtained.

A method of fabricating the magnet system is also provided in thedisclosure of the prior art. According to the method, a uniform magneticfield in a sheet outside the magnet is obtained and the gradient fieldalong two orthogonal directions except for slice thickness direction isproduced in the volume of interest. Referring to FIG. 2, the magnet 10consists of field yoke 11, the first magnetic stack 12, the secondmagnetic stack 13, the first gradient coil 15 and the second gradientcoil 16. The two magnetic stacks 12/13 are placed on the field yoke 11symmetrically relative to the field yoke 11. There is an angle betweenthe surfaces of the two magnetic stacks 12/13 facing to the bed. Aferric plate 17 for adjusting the field is placed between the twomagnetic stacks 12/13, which can change the distribution of the magneticfield in the area 1 of interest and make the uniformity of the magneticfield meet the requirement. The first gradient coil 15 and the secondgradient coil 16 are superposed on the surfaces of the two magneticstacks 12/13 facing to the bed, respectively. Furthermore, for the firstand second magnetic stacks 12/13, the angle θ between the surfacesfacing to the bed is preferably between 0 and 180 degree. The magneticstacks are made from many kinds of permanent magnetic materialsoverlapped in layers, such as permanent magnetic ferrite, alnico,samarium cobalt alloy, sintered NdFeB, forming a wedge form. One sidesurface of the wedge is attached to the magnetic yoke and another facingto the volume of interest. The magnetization direction of the magneticmaterial is along the direction from one surface of the wedge toanother.

Furthermore, for the first and second magnetic stacks 12/13 made ofsingle magnetic material, the angle between the surfaces facing to thebed is preferably zero. The shape of these two magnetic stacks is strip.Furthermore, the shape of the ferric plate 17 for adjusting the field isstrip. Its cross section is anyone of the following shapes: square,rectangle, trapezoid, half circle and circle. The ferric plate 17 isfixed on the joint between the first and second magnetic stacks 12/13with glue joint or mechanical bonding. If these two magnetic stacks areseparated, the slab is fixed on the yoke between these two magneticstacks.

Furthermore, the first and second gradient coils 15/16 in the magnetsystem which produce linear gradient field along two orthogonaldirections in the volume of interest except for the slice thicknessdirection are fixed on the surface of the first and second magneticstacks 12/13 facing to the bed. These two gradient coils with the sameshape and number of turns could be overlapped on the magnetic stacks.

Furthermore, in this full open MRI scanner, the bed is made fromnon-conductive and non-ferromagnetic materials and placed under thevolume of interest. The surface of the bed is parallel to the plane ofthe sheet of interest and lower than it. The altitude of the bed can beadjusted by the servocontrol system manipulating the lift stand on theback of the bed.

According to this technique, the volume of interest is on the one sideof the magnet, which permits full access to the patient. This kind ofMRI system can be applied in the interventional therapy. In thisinvention, the volume of interest is not a large sphere but a sheetoutside of the magnet, which makes the magnet light and small. The MRIsystem can be made compact and movable.

In the above-mentioned full open magnetic resonance imaging apparatus,two symmetrical magnetic stacks and a ferric plate for adjusting thefield are adopted to form a uniform and parallel magnetic field for thefirst time. But, the formation and the adjusting method for the magnetare theoretical and not concrete, thus it affects the application of theabove-mentioned MRI system.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a magnet for MRIscanner, which can produce a uniform sheet of interest by improving thecooperation between the permanent magnets and the field yokes and usingadjusting block of two layers. In the volume of interest, magneticinduction intensity B has the identical direction and the inhomogeneityof its module is within the order of no more than 10⁻⁴-10⁻⁵ which canmeet the requirement of MRI.

The present invention provides a magnet for MRI system, comprising afield yoke for forming flux path; a first member of permanent magnet,provided on one end of the field yoke, free end of which is inclinedinwardly; a second member of permanent magnet, provided on the other endof the field yoke in symmetry with the first member of permanent magnet,having reverse polarity with the first member of permanent magnet; anadjusting block, provided on the center of the field yoke for extrudingmagnetic field formed by the first and second members of permanentmagnet. The adjusting block is divided into two parts to be symmetricaleach other, each of which has identical polarity with the first/secondmember of permanent magnet to be adjacent. Each part of the adjustingblock comprises a slab of permanent magnet, a magnetic conductorprovided on the field yoke for supporting the slab of permanent magnet.

The free end of each member of permanent magnet comprises a first endface, a second end face perpendicular to the first end face. The widthof the first end face is larger than that of the second end face.Furthermore, an angle between the first/second member of permanentmagnet and the field yoke is about 25°˜55°; an angle between themagnetization direction and an axial of each member of permanent magnetis about −20°˜20°. Preferably, each member of permanent magnet isdivided into several subsections, each of which is individuallymagnetized, and an angle between magnetization direction and an axial ofthe first/second member of permanent magnet for each subsection is ofincrement in turn from fixed end to the free end.

The member of permanent magnet comprises a base; side parts, coupled tothe base respectively, the maximum magnetic energy level of which islarger than that of the base. The length of the base is about 2˜6 timesof that of the side part.

For the application, the distance between the free ends of the first andsecond members of permanent magnet is about 400 mm, and the distancebetween the free end and the adjusting block is bout 200 mm.

The present invention also discloses a method for forming uniformmagnetic field in magnetic resonance imaging system, comprising the stepof forming a field yoke for forming flux loop; forming a first member ofpermanent magnet, provided on one end of the field yoke, free end ofwhich is inclined inwardly; forming a second member of permanent magnet,provided on the other end of the field yoke in symmetry with the firstmember of permanent magnet, having reverse polarity with the fist memberof permanent magnet; forming an adjusting block, provided on the centerof the field yoke for extruding magnetic field formed by the first andsecond members of permanent magnet.

The present invention has the following advantages:

The magnet is light and with a high open degree because a large uniformmagnetic field volume is not required (generally a sphere with a 30˜50cm diameter). Thus, the magnet provided in this invention is especiallyuseful for MRI system for operations and makes the MRI system very cheapand moveable.

A new imaging method is presented in this invention. Slice selection iscanceled and the objective slice in the examined tissue is directlyplaced in a sheet of interest rather than a sphere to obtain the imagesdirectly.

The servocontrol system is included in the MRI system for moving the bedand making the different objective slices of the tissue overlap with thesheet of interest.

More importantly, in the MRI system according to the present invention,a uniform sheet of interest is obtained in which the inhomogeneity ofthe magnetic field is below the order of 10⁻⁴-10⁻⁵, and the requirementof MRI is met. The magnet is light and with a high open degree because alarge uniform field volume is not required (generally a sphere with a30˜50 cm diameter) in this magnet apparatus. Thus, the magnet in thisinvention is especially useful for MRI system for operations and makesit very cheap and moveable.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects of the invention will be apparent from thefollowing detailed description of the embodiments of the presentinvention with reference to the accompanying drawings, in which:

FIG. 1 is a schematic view of an open MRI system according to the priorart;

FIG. 2 is a perspective view of the magnet apparatus in the system ofFIG. 1;

FIG. 3 is a schematic view of an open MRI system of the presentinvention;

FIG. 4A is a schematic view of the magnet according to a preferredembodiment of the present invention;

FIG. 4B is a schematic view of the magnet according to another preferredembodiment of the present invention;

FIG. 5 is a perspective view of the members of permanent magnet of thepresent invention;

FIG. 6 is a schematic view of the end of the members of permanent magnetshown in FIG. 5, illustrating the magnetization directions of themembers;

FIG. 7 is a schematic view of the members of permanent magnet shown inFIG. 5 which are magnetized by dividing into subsections;

FIG. 8 is a schematic view of the distribution for magnetic line offorce of the magnetic field produced by the members of permanent magnetmatched with the field yoke;

FIG. 9 is a schematic view of the distribution for magnetic inductiveintensity in the open area between the free ends of the members ofpermanent magnet shown in FIG. 8;

FIG. 10 is a schematic view of the distribution for magnetic line offorce in the magnetic field produced by the adjusting slab;

FIG. 11A is a schematic view of the distribution for magnetic line offorce of the preferred embodiment of the present invention above shownin FIG. 4A;

FIG. 11B is a schematic view of the distribution for magnetic line offorce of the preferred embodiment of the present invention above shownin FIG. 4B;

FIG. 12 is a schematic view of the distribution for magnetic inductiveintensity in the open area between the free ends of the members ofpermanent magnet by adding the adjusting block in the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings. As shown in FIG. 3 which is a schematic view of the open MRIsystem of the present invention, the MRI system comprises a magnet 100(comprising magnet and gradient coils), servocontrol system for movingbed, main controller, RF controller, RF amplifier, RF coils, gradientcontroller, preamplifier, receiver, image reconstruction part andcomputer. Except for the magnet, all other parts published before willnot be described here.

In order to acquire a uniform magnetic field in a sheet, the base fieldis produced by three parts which is extruded by an adjusting fieldproduced by a group of adjusting parts. As shown in FIG. 4A, the magnet100 of one preferred embodiment of the invention comprises a field yoke110 for forming flux path, a first member of permanent magnet 120 and asecond member of permanent magnet 120′, a first adjusting block 130 anda second adjusting block 130′. The first and second members of permanentmagnet 120/120′ are respectively provided on the two ends of the fieldyoke 110 in symmetry relative to the central line of the field yoke 110,the free ends 122/122′ of which having reverse polarity is inclinedinwardly and are separated from each other to form an open area A. Thefirst and second adjusting block 130/130′ are symmetrically provided onthe field yoke 110 and between the two free ends 122/122′ Specially, theadjusting block of the present invention is of double layers. Each ofthe adjusting block 130/130′ consists of a slab of permanent magnet132/132′, a magnetic conductor 134/134′ which is provided on the fieldyoke 110. The magnetic conductor 134/134′ can support the slab ofpermanent magnet 132/132′ and help to form the flux loop to adjust thefield in the open area A.

To extrude the magnetic field in the open area A, according to the truththat the identical poles always repel and the opposite poles alwaysattract, the first part of adjusting block 130 and the first member ofpermanent magnet 120, the second part of adjusting block 130′ and thesecond member of permanent magnet 120′, respectively have the identicalpolarity. In the preferred embodiment shown in FIG. 4A, the shape of thetwo parts of adjusting block 130/130′ which are symmetric relative tothe normal line of the field yoke 110 is wedge. In the preferredembodiment shown in FIG. 4B, the shape of the two parts of adjustingblock 232/232′ is cuboids. According to the present invention, the angleα between the field yoke 110 and the two members of permanent magnet120/120′ is within the range from 25 to 55 degree. Preferably, the angleα is about 44 degree.

The detailed description of the members of permanent magnet is givenbelow referring to the first member of permanent magnet 120. As shown inFIG. 5, the first member of permanent magnet 120 along the lengthdirection comprises a base 123, two side parts 125 which are coupled tothe base respectively. Because the magnetic field of the side parts 125could be weakened by the leakage of magnetism, the side parts 125 aremade from materials of permanent magnet with maximum energy levelBH_(max) higher than that of the base 123. Preferably, the length of thebase 123 is 2˜6 times of that of the side parts 125. Furthermore, thefree end 122 consists of a first end face 124 and a second end face 126.The second end face 126 is almost perpendicular to the first end face124, so that the magnetic field in the open area A can be adjustedlittle by little to get the perfect field. Surely, the width of thefirst end face 124 can be equal to that of the second end face 126.According to the optimized design, the width of the first end face 124is larger than that of the second end face 126. Preferably, therelationship between the first end face 124 and the second end face 126is to meet the following equation: $\frac{W}{W + H} = 0.618$wherein W is representative of the width of the first end face 124 and His representative of the width of the second end face 126.

FIG. 6 is a schematic view of the end of the members of permanent magnetin the present invention. In fact, the magnetization directions of eachmember of permanent magnet have significant relations with the magneticfield in the open area A. To form a uniform field, according to theoptimized design in the present invention, the angle between themagnetization direction and the axial of each member of permanent magnetis about −20°˜20°. Furthermore, according to the present invention, eachmember of permanent magnet is divided into several subsections, each ofwhich is individually magnetized. As shown in FIG. 7, from I₁ to I_(n),the angle Φ_(i) between magnetization direction and the axial of thefirst/second member of permanent magnet for each subsection is ofincrescent in turn from the fixed end to the free end.

In the present invention, the distance between the free ends 122/122′ ofthe first and second member of permanent magnet is about 400 mm, whichis very convenient for the interventional therapy. Meanwhile, thedistance between the free end 122 and the adjusting block 130 is bout200 mm, which is convenient to place the imaging sample.

After removing the adjusting block, the distribution for magnetic lineof force in the magnetic field produced by the members of permanentmagnet matched with the field yoke is shown in FIG. 8. In the open areabetween the free ends 122/122′ of the first and second member ofpermanent magnet, the magnetic line of force of the second end face isbecoming uniform but not perfect. Seen from the graph of magneticinductive intensity-distance shown in FIG. 9, the area of the uniformmagnetic field is too small to meet the requirement.

FIG. 10 is a schematic view of the distribution for magnetic line offorce in the magnetic field produced by the adjusting block. As theadjusting block in the present invention is of double layers and fluxloop can be formed between the first and second slabs of permanentmagnet and the magnetic conductors, a regular magnetic field can beproduced outside the adjusting block i.e. between the parts of theadjusting block and in the area near the outer of adjusting block. Theaction of repelling between the magnetic field above and that in theopen area A can extrude the latter, and further a uniform magnetic fieldshown in FIG. 11A is obtained. Similarly, the curves shown in FIG. 11Bare representative of the distribution for magnetic line of force in themagnetic field produced by the preferred embodiment shown in FIG. 4B,illustrating the uniform field between the free ends.

The graph of magnetic inductive intensity-distance got from FIG. 11A isshown in FIG. 12. This shows a uniform magnetic field with proper widthcan be obtained by adding the adjusting block in the present invention.According to the present invention, the thickness of the uniformmagnetic field is about 3 mm.

1. A magnet for magnetic resonance imaging system comprising a fieldyoke for forming flux loop; a first member of permanent magnet, providedon one end of the field yoke, free end of which is inclined inwardly; asecond member of permanent magnet, provided on the other end of thefield yoke in symmetry with the first member of permanent magnet, havingreverse polarity with the first member of permanent magnet; an adjustingblock, provided on the center of the field yoke for extruding magneticfield formed by the first and second members of permanent magnet.
 2. Amagnet according to claim 1, wherein the adjusting block is divided intotwo parts to be symmetrical each other relative to the centerline of,each of which has identical polarity with the first/second member ofpermanent magnet to be adjacent.
 3. A magnet according to claim 2,wherein each part of the adjusting block comprising a slab of permanentmagnet; a magnetic conductor, provided on the field yoke for supportingthe slab of permanent magnet.
 4. A magnet according to claim 3, whereinthe slab of permanent magnet is wedge shape.
 5. A magnet according toclaim 3, wherein the slab of permanent magnet is cuboids.
 6. A magnetaccording to claim 1, wherein the free end of each member of permanentmagnet comprises a first end face; a second end face perpendicular tothe first end face.
 7. A magnet according to claim 6, wherein the widthof the first end face is larger than that of the second end face.
 8. Amagnet according to claim 7, wherein the relationship between the widthsof the first end face and second end face is $\frac{W}{W + H} = 0.618$wherein W is representative of the width of the first end face and H isrepresentative of the width of the second end face.
 9. A magnetaccording to 6, wherein an angle between the first/second member ofpermanent magnet and the field yoke is about 25°˜55°.
 10. A magnetaccording to claim 9, wherein an angle between the first/second memberof permanent magnet is about 44°.
 11. A magnet according to claim 9,wherein an angle between the magnetization direction and an axial ofeach member of permanent magnet is about −20°˜20°.
 12. A magnetaccording to claim 11, wherein each member of permanent magnet isdivided into several subsections, each of which is individuallymagnetized.
 13. A magnet according to claim 12, wherein an angle betweenmagnetization direction and an axial of the first/second member ofpermanent magnet for each subsection is of increment in turn from fixedend to the free end.
 14. A magnet according to claim 6, wherein eachmember of permanent magnet comprising a base; side parts, coupled to thebase respectively, the maximum magnetic energy lever of which is largerthan that of the base.
 15. A magnet according to claim 14, wherein thelength of the base is about 2˜6 times of that of the side part.
 16. Amagnet according to claim 15, wherein the distance between the free endsof the first and second members of permanent magnet is about 400 mm. 17.A magnet according to claim 16, wherein the distance between the freeend and the adjusting block is bout 200 mm.
 18. A method for forminguniform magnetic field in magnetic resonance imaging system, comprisingforming a field yoke for forming flux path; forming a first member ofpermanent magnet, provided on one end of the field yoke, free end ofwhich is inclined inwardly; forming a second member of permanent magnet,provided on the other end of the field yoke in symmetry with the firstmember of permanent magnet, having reverse polarity with the fist memberof permanent magnet; forming an adjusting block, provided on the centerof the field yoke for extruding magnetic field formed by the first andsecond members of permanent magnet.
 19. A method according to claim 18,wherein steps of forming each part of adjusting block comprising forminga slab of permanent magnet, which has identical polarity with thefirst/second member of permanent magnet to be adjacent; forming amagnetic conductor, provided on the field yoke for supporting the slabof permanent magnet.
 20. A method according to claim 19, wherein stepsof forming each member of permanent magnet comprising forming a firstend face at the free end of each member of permanent magnet; forming asecond end face perpendicular to the first end face at the free end ofeach member of permanent magnet.
 21. A method according to claim 20,wherein the angle between the first/second member of permanent magnetand the field yoke is about 25°˜55°.
 22. A method according to claim 21,wherein the angle between the magnetization direction and an axial ofeach member of permanent magnet is about −20°˜20°.
 23. A methodaccording to claim 22, wherein each member of permanent magnet isdivided into several subsections, each of which is individuallymagnetized.
 24. A method according to claim 23, wherein the anglebetween the first/second member of permanent magnet is about 44°
 25. Amethod according to claim 18, wherein along the length direction of eachmember of permanent magnet forming a base; forming side parts, coupledto the base respectively, the maximum magnetic energy lever of which islarger than that of the base.
 26. A method according to claim 25,wherein the length of the base is 2˜6 times of that of the side parts.27. A method according to claim 26, wherein the distance between thefree ends of the first and second member of permanent magnet is about400 mm.